Organize Grouping Structures within a Canvas

_From @cubap on October 16, 2015 5:50_

Group and order lines within a column (currently just xMin,yMin to xMax,yMax) and order columns on a page. It may be possible or pleasant to allow infinite recursion with this so the group is just a `sc:Range`-like structure to organize an ordered list of annotations or of other lists. Each of these should also have their own label.

_Copied from original issue: cubap/TPEN3#4_